Andrew Ward / Maricopa County Sheriff's Office

by Vianka Villa and Randsom Rockliffe, The Arizona Republic

by Vianka Villa and Randsom Rockliffe, The Arizona Republic

PHOENIX -- A Maricopa County inmate charged in the fatal stabbing of his 12-year-old brother in Phoenix now stands accused of killing his cellmate in a frenzied attack on Wednesday night.

Andrew Ward, 27, was arrested early Thursday on suspicion of killing Douglas William Walker, who was awaiting sentencing on an armed robbery conviction, according to the Maricopa County Sheriff's Office.

Sheriff's officials said Walker was found "beaten, stabbed with a golf pencil and smeared with peanut butter over his head." He was discovered at about 7:30 p.m. when inmates notified detention officers on a security walk of a fight inside the cell.

Phoenix fire paramedics pronounced Walker dead at the scene. Paramedics also determined that a plastic bag had been placed in Walker's nose and throat and obstructed his breathing.

Ward reportedly admitted to a play-by-play of the attack in an interview with detectives and told investigators that he had "no regrets," according to a sheriff's statement.

Ward relayed that he had cut Walker's throat with a plastic playing card, stabbed him in the eyes and throat with a golf pencil and finished the assault by stuffing a plastic bag down Walker's throat, according to a sheriff's statement.

Sheriff's Office spokesman Chris Hegstrom said Ward has been re-classified and has his own cell at the jail.

Both Ward and Walker were placed in segregated custody in the county jail system after each told jail administrators they feared for their safety, according to the Sheriff's Office.

Walker pleaded guilty to armed robbery charges last month and was due to be sentenced - and likely transferred to the Department of Corrections - on April 11.

Ward was arrested March 12 on suspicion of stabbing and killing his 12-year-old brother in a Phoenix home last month. He pleaded not guilty to the allegations in a brief court hearing.

Police said Ward called 911 on March 12 from a convenience store and reported he had stabbed someone at a house in Phoenix.

Officers found Austin Tapia with multiple and fatal stab wounds when they arrived at the home at about 5:30 p.m.

Ward had blood on his clothing and was believed to be carrying a knife in his pants pocket when he was taken into custody at the convenience store, said Sgt. Steve Martos, a Phoenix police spokesman.

Police said Ward was alone with his brother, whose mother and two sisters were out to dinner. Austin had decided to stay home.

Detectives said that when they asked Ward why he killed his brother he told them, "Honestly, I just felt like killing."

In filing the probable-cause statement, police suggested Ward may be an addict and mentally ill and had asked to "go to (a) mental hospital" instead of jail when he called police.

The report said Ward's family reported that he suffered from depression and had a history of domestic violence in the home.

The family also said Ward had threatened them in the past and that his siblings had called the police on him before.